The present invention relates to a hydraulic booster for amplifying a manual control force by producing an auxiliary dynamic pressure, and more particularly to a hydraulic booster for amplifying the output of a hydraulic master cylinder as a braking force generator of an automobile.
A hydraulic booster of this type is adapted to apply a dynamic pressure on the backside of a power piston after controlling it to a level proportional to the force applied to the brake pedal by opening and closing its valve unit. The valve unit is opened or closed according to the relative position between an input rod and the power piston. Namely, when the power piston is retracted relative to the input rod, the pressure of the dynamic pressure source is introduced into the dynamic pressure chamber to increase the dynamic pressure output. When the power piston is advanced relative to the input rod, the dynamic pressure chamber is opened to the reservoir to depressurize the dynamic pressure chamber.
In one conventional arrangement, this control is carried out by a valve unit provided in the power piston so as to move together with the power piston and the input rod. In another arrangement, the relative position between the piston and the rod is detected and transmitted to a fixed valve unit provided separate from the piston and the rod.
Of these two arrangements, the latter requires a completed and expensive link mechanism. Thus, most of conventional boosters employ the former arrangement.
In the former arrangement, it is necessary to introduce the pressure of the dynamic pressure source into the axially movable power piston. There are two known ways to do this. One is to connect the outlet of the dynamic pressure source with the valve chest formed in the power piston by means of a flexible hose. The other is to provide an annular fluid pressure introducing chamber around the power piston so as to communicate with the valve chest through a hole formed in the power piston. In the latter method, it is necessary to provide high-pressure seals on both sides of the chamber to seal the chamber while allowing sliding movement of the piston. Of these two methods, the latter is generally preferred to the former because the flexible hose used in the former method has durability-related problems.
But the latter method is not without problems, either. Namely, since the fluid pressure introducing chamber is in direct fluid communication with the dynamic pressure source, the two seals defining the chamber are always subjected to high pressure equal to the pressure of the dynamic pressure source. This necessarily leads to increased sliding resistance of the seal, which in turn makes it necessary to use a stronger return spring to return the power piston to its original position. Thus, when the brake pedal is depressed, the power piston will not move soon, but begins to move only after the force applied to the piston exceeds the sum of the sliding resistance of the seals and the force of the return spring. This impairs the brake pedal feeling in the initial stage of its stroke. Moreover, since the seals are forced to slide while being subjected to high pressure equal to the pressure of the dynamic pressure source, they have to be formed from a sufficiently durable material. Durable material is usually not only expensive but tends to incur further increase in the sliding resistance of the seals.
One solution to this problem is proposed in EP 296614 B1. The hydraulic booster disclosed in this publication has a pressure control valve provided between the dynamic pressure source and the fluid pressure introducing chamber to keep the pressure in the latter chamber higher by a predetermined level than the pressure in the dynamic pressure chamber by opening and closing the passage connecting the dynamic pressure source with the chamber.
As such pressure control valves, the publication 64-1652 discloses a spool valve and another valve having a check valve and a piston carrying a pin adapted to push up the check valve. The valve of the latter type is also known from FR-A 2604673 and U.S. Pat. No. 4,463,561.
A spool valve has a problem in that it is difficult to prevent leakage of fluid while the valve is closed. In order to prevent fluid leakage so that it can be used as a pressure control valve, its piston and piston case have to be finished to extremely narrow tolerances so that there will be practically no clearance therebetween. Such precise machining is, however, practically impossible. Thus, we studied a hydraulic booster including the valve of the latter type. But this booster also turned out to be practically useless because it frequently caused vibrations of the brake pedal and abnormal noise when the brake pedal is depressed.
FIG. 1 shows a hydraulic booster disclosed in EP 296614 B1. It comprises a body 2 having a bore; a power piston 1 axially slidably mounted in the bore of the body 2; an input rod 3 provided behind the power piston 1; a spool valve 4 axially slidably mounted in a valve chest formed in the power piston 1; a dynamic pressure chamber 5 for applying dynamic pressure on part of the backside of the power piston 1; a fluid pressure introducing chamber 6 provided between the bore of the body 2 and the outer periphery of the power piston 1 and connected to an output circuit of a pump 11; high-pressure seals 7, 8 provided in the front and rear of the chamber 6 to seal it while allowing sliding movement of the piston 1; a return spring 9 for the power piston 1; a return spring 10 for the spool valve 4; a reservoir 12; and an accumulator 13. Numerals 14, 15 and 16 indicate holes, 17 a depressurization passage, and 18 a fluid passage. Numeral 30 designates a pressure control valve. It includes a piston 36 carrying a push pin 35, an offset spring 34 biasing the piston 36 toward a chamber 32, and a ball valve 37 for opening and closing the passage connecting the dynamic pressure source with the chamber 32. The ball valve 37 is moved between its open and closed positions by reciprocating the piston 36.
The relation of forces produced in the pressure control valve 30 is given by the following formula: EQU F+P.sub.3 .multidot.A=P.sub.1 .multidot.B+P.sub.2 .multidot.(A-B)(1)
wherein A is the sectional area of the piston 36; B is the sealing area of the ball valve 37; P1 is the pressure of the dynamic pressure source; P2 is the pressure in the fluid pressure introducing chamber 6; P3 is the pressure in the dynamic pressure chamber 5; and F is the force of the spring 34. The value B can be made substantially smaller than the value A. Thus, P2 =P3 +F/A. Immediately after the brake pedal is depressed, the pressure P3 in the dynamic pressure chamber 5 is still small, so that the pressure P2 in the chamber 6 is substantially equal to F/A. For example, if the pressure P1 in the dynamic pressure source is 160 kg/cm.sup.2, F/A =10-20 kg/cm.sup.2. Thus, the slide resistance is kept sufficiently low.
But actually, one cannot ignore the influence of the pressure P1 of the dynamic pressure source. Namely, the pressure control valve 30 opens when the formula (1) is met. The moment the valve 30 opens, the flow speed of the fluid present between the pump 11 and the pressure control valve 30 increases sharply, so that the pressure at the inlet of the pressure control valve, which is equal to the pressure P1 of the dynamic pressure source, drops sharply to the level substantially equal to P2. The longer the pipe between the pump 11 and the pressure-control valve 30, the more quickly the pressure at the inlet of the valve 30 tends to drop due to the influence of the inertia and viscosity of the fluid in the pipe. When the pressure at the inlet of the valve 30 drops to P2, the following relation is met: EQU F+P.sub.3 .multidot.A=P.sub.2 .multidot.A (2)
In this state, the pressure control valve 30 shows a tendency to increase the pressure P2, i.e. a tendency to open. Thus, the moment the valve 30 begins to open, the valve-opening tendency intensifies. As a result, the pressure at the inlet of the pressure control valve decreases still further. The valve closing force overcomes the valve opening force only after the pressure P2 increases to a level at which the formula (2) is met. In this state, the pressure at the inlet of the pressure control valve increases to the level equal to the pressure P1 of the dynamic pressure source. Now, the pressure control valve is kept closed until the pressure at the inlet of the pressure control valve 30 decreases to the level equal to the pressure P2, i.e. the level at which the formula (1) is met. Thus, the pressure control valve swings between the states represented by the formulas (1) and (2). This unstable state of the valve 30 causes vibrations of the spool valve 4 and thus the vibrations of the brake pedal and abnormal noise in the-booster.
An object of the present invention is to provide a hydraulic booster which has a pressure control valve and is free of vibration and abnormal noise.